Since ethylene-vinyl alcohol copolymer possesses a high oxygen barrier property and exhibits excellent processing property and solvent-resistance, it has been widely used in various applications such as food package, chemical solvent package, preserving the aroma of cosmetic perfume, recently. However, during the processing of EVOH, EVOH should pass through an extruder at an elevated temperature to be molten so that EVOH should undergo an elevated temperature for a period (usually at a processing temperature of from 200 to 240° C.). Under the processing condition, EVOH will become serious yellowing in quality if no additive for increasing heat-resistance is added. It also experimentally demonstrated that EVOH having been added with such additional additives exhibits a better heat resistance and can withstand a higher temperature.
Such additives are conventional added into dried EVOH copolymer by blending means, which certainly would produce a non-uniform mixture and in turn result in yellowing on part of the processed product. Therefore there are many proposals to improve EVOH's heat-resistance. For example, Japan Patent Examined Publication Sho 46-37664 disclosed treatment of EVOH with various acids such as acetic acid, formic acid, and tartaric acid. The prepared EVOH in Sho 46-37664 possesses different decomposing temperature, which is a temperature for achieving 5% by weight heat-loss. Japan Patent Examined Publication Sho 55-19242 disclosed a method for improving heat-resistance of EVOH by adding a mixture of acetic acid and phosphoric acid. Japan Patent Examined Publication Sho 57-5834 disclosed incorporation of a mixture of organic acids and their sodium or potassium salt to increase heat-resistance of EVOH. In the examples of Sho 57-5834, it used a mixture of sodium benzoate and potassium benzoate, and of adipic acid and sodium adipate. However, since benzoates contain a benzene ring, incorporation of it into EVOH will cause the EVOH not suitable for food package field.
However, the heat-resistance of EVOH produced by the above processes is still not satisfactory. There still requires a method for further increasing the heat-resistance of EVOH.
Under this circumstance, the present inventors have conducted an investigation on EVOH properties and found that before drying, EVOH is a porous material. By utilizing such a porous property, incorporation of additives into aqueous solution in acid-treating step prior to drying will allow the additives disperse in the pores uniformly, thereby a heat-resistance of EVOH increases and the resulting product exhibits uniform heat-resistance.
The present invention has been completed based on the above finding.